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autolux
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Here's my ordeal:

I purchased a 74' R90 last November. Ran fine for about a week. Soon after it was nearly impossible to cold start, would take 10-15 minutes of trying. I left for tour and it stayed in the garage for a few months, I return home and it turns over, but won't start.

So I began troubleshooting: didnt have a spark, so I replaced the points and condenser, and threw in new spark plugs as well. I did everything per the clymer manual. One odd thing that was happening, when I was trying to set the static ignition timing, using a multimeter I was getting constant continuity as I rotated the engine. No matter where I am on the timing belt I get a beep.

So now, I get a spark and it turns over, but won't fire. And i'm getting a shitload of backfiring when I'm trying to start it.

As of now, my next step is to check compression and adjust the valves. Any help would be greatly appreciated.

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schrader7032
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Well, for an engine to run,

Well, for an engine to run, you need fuel, compression, and spark (at the right time). You can look in the float bowls to see if there's about an inch of fuel in there. You could remove the spark plug, put your thumb over the hole, and turn the engine by hand...do you feel compression against your thumb? Valves would be good to check.

My guess is that you don't have spark at the right time. Try this. Take the spark plugs out and put them back in the caps...lay the plugs on the engine and ensure that the threads are grounded. Remove the battery ground, removed the front cover, reinstall the battery ground. Turn the ignition on...don't start. Now with an allen wrench in the end of the alternator, rotate the engine (CW when viewed from the front if I recall correctly) and watch the timing marks. Do you hear the spark plugs snap/fire each time the S-mark shows up in the window?

Hope you find the problem.

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

mark_weiss
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The condenser provides a

The condenser provides a constant path to ground, so if you are checking for the points to open using an ohm meter, you will indeed see constant continuity. You could take the condenser out of the circuit while setting the timing or you could switch to checking for voltage. The condenser will have negligible effect on a voltage check. You can also use a test lamp. Attach one lead to either side of the dual terminal at the condenser. Ground the other lead. With the ignition switched on, check your set up by manually opening the contact points. The voltmeter should move decisively. If not, use a screwdriver or wire to short the two sides of the contacts to each other. You just want to be sure that the points really are opening and closing and not simply moving back and forth without making contact. Since you note that you are getting spark, your contact should be making, and breaking, contact.

Once that is settled, rotate the engine and stop exactly at the point where the test lamp illuminates or the voltmeter jumps to read battery voltage. Now, take a look at the timing marks and see where you are. Next, adjust appropriately.

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autolux
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I just finished adjusting the

I just finished adjusting the valve clearances to spec, both sides were incredibly tight. I thought that was going to at least get it to crank... still the same though.

I'm gonna try both of your suggestions on getting the spark timed up tomorrow, and I'll get back to you.

My main concern is that there's something major going on in the top end. The engine occasionally seizes when I'm trying start it (along with the ungodly amount of backfiring). The seizing just started recently. I've had troubles cold starting it since the beginning though.

Thanks again dudes.

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autolux
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@Schrader & @mark

THANK YOU!!!!!

finally got the spark to land on the S after an hour or two of adjusting. I believe my ATU should be replaced soon. But she cranked right up!

Im getting a lot more valve noise than I was, but I adjusted the clearances to spec, maybe just slightly loose. I didn't adjust the rocker arms, which I might go back and do here shortly.

Thanks again fellas. I was getting close to givin' up there for a second.

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schrader7032
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It is possible to adjust the

It is possible to adjust the valve clearances in such a way that results in very wide clearances. It's important to adjust the clearance on the side that is at OT on the compression stroke. Then the engine must be turned 360 degrees so that the other cylinder is at OT on the compression stroke. So often I see people write that "find the side where the pushrods can be turned by hand, that's the side to work on." That has an inherent flaw in it because if you happen to have loose clearances to start with, you'll be way off when setting them.

I make sure I'm doing the correct side by removing the valve cover, putting the trans in 5th gear, and manually rotating the rear wheel while watching the valve train. When I see the intake valve open and then close, I sit down by the timing hole and continue to turn the rear wheel. The next OT is where I want to set the valves for the side I was watching. Repeat for the other side.

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

mark_weiss
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Tight valves WILL yield a

Tight valves WILL yield a quieter running engine. For a while...
Noise is normal. If you are concerned, it is not too difficult to go back and check your work. Be sure to have valve clearance before checking rocker arm end float. There should be some perceptible movement (enough to squish oil out between the rocker pivot and the support block, but not so much that the rocker arm can clatter up and down. The upper and lower blocks are movable, should not take much to clear up excess play. Remember that the pushrod pushes the rocker upward and that the rocker pad should contact the valve stem somewhat off-center. After you are satisfied with the rockers, re-check the valve lash. If you are going to err, err on the loose side. More time on the valve seat is better for the valves, especially the exhaust valve.

ATUs rarely wear out. Bring up the revs and see that the dot above the F mark is approximately centered in the window. This is where you will be running most of the time anyway, so if there's error in the timing, full advance is what you want to have correct. With the F end set, check that the idle mark centers at idle. If the advance slots are worn, the timing will be a bit too retarded at idle. This is not really a major problem. The advance springs DO indeed wear out and need periodic replacement. The 'sport' springs are a good choice because they delay the rpm at which you will reach full advance and this is more kind to your engine.

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autolux
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I have one last issue to

I have one last issue to solve.

So she's cranks up perfect, at idle S dead center of the window, & at 3k rpm F dot at the center of the window.

But anytime I roll on a decent amount of throttle the engine spits and farts. The threads on the nut of the ATU were stripped, I replaced the nut & reseated the ATU but the same thing is happening still. Could it be the springs on the advance causing this? or the entire unit itself? I did notice the the large felt pad was so worn down it's not even touching anymore.

Or could this be a carb issue? although thats not what it feels like.

And should I avoid riding the bike until I get the spitting & farting fixed?

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schrader7032
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Sounds like a carb synch is

Sounds like a carb synch is needed...or basic carb work. How did you put another nut on the end of the shaft if the threads are stripped? If the threads are gone or sheared off, about the only easy solution is to substitute a Boyer ignition system which replaces the ATU and uses a computer controlled advance curve.

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

autolux
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the threads of the nut were

the threads of the nut were stripped, the shaft itself was fine. The new nut threaded on perfectly

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Darryl.Richman
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With the front engine cover

With the front engine cover off, in a darkened garage, point your timing light at the advance unit and roll on and off the throttle. You should be able to see the arms swing out and retract smoothly.

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autolux
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It's running now, but running

It's running now, but running awfully. I cleaned and greased the ATU & camshaft, one of the springs appears to need replacing but other than that, the advance SEEMS to be doing its job. At idle the S is dead center, & the F doesn't move past center at 3k & above. I'm getting some dual imaging, but not too serious, which I read is common.

but the engine spitting is awful. I can barely apply throttle. It's either dies at idle, or idles super fast around 1.5k - 2k rpm.

I pulled the plugs (brand new bosch W6DC's) and they were white & burnt. Could that be from all of the ignition timing adjustment i've been doing? or is something else going on? I know that means the engine is running hot. I put the NGK's back in.

Kurt, I'm going to pull apart both carbs tomorrow & do a full clean.

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Darryl.Richman
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Sounds like you got a bad

Sounds like you got a bad condenser. Try putting the original one back in or buy another new one and try that. If you run the motor in a darkened garage with the front cover off, do you see the points arcing badly, especially as you try to rev it up?

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autolux
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I'll try that today before I

I'll try that today before I take the carbs apart.

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autolux
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Started working on the carbs

Started working on the carbs today and discovered that she's only been running on one cylinder. No spark at the left cylinder. This is odd, because I've been using the left plug as my timing reference. I seemed to be getting a decent & somewhat consistent spark last week, but today there was no spark at the left.

I also reset my points gap & timing today.

When I have the cap off of the left plug, the right side cranks right up, idles smoothly, and gives great throttle response.

Guessing that my left ignition coil is dead?

Darryl, I switched out condensers & there was no perceivable difference between the old one and the new.

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schrader7032
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I would recommend that you

I would recommend that you don't pull a spark plug cap and either crank the engine or start and run it. If the spark energy can't find a way to ground, it will bounce and rebound with possible damage to the coils. It might not show up now, but could later on. This is also especially harmful for electronic ignition systems.

Look into the coil towers...if there's corrosion in there, it needs to be cleaned out. Try measuring the resistance from one spark plug cap to the other. The measurement should be 20K-22K ohms. Generally that's about 5K ohms for each spark plug cap (you're might only be 1K ohm caps) plus another approximate 5K for each coil. If you don't get those kinds of readings, then you could have a coil issue.

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'78 R100/7 '69 R69S '52 R25/2

mark_weiss
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Ignition coils very, very,

Ignition coils very, very, rarely (but not never) die. Far more common is a spark plug cap with high resistance (should be 5k ohms) or a damaged high tension wire. Swap the wire and cap to the other side and see if the problem moves too.

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autolux
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My left high tension wire

My left high tension wire turned out to be bad, was reading in the megaohm range. The right one was fine, reading 5k

Tested my coils as well they both metered around 5 ohms.

Which I was surprised to see 12v coils. I was under the impression that these bikes ran on two 6v coils in series? Does this mean my two 12v coils share voltage and only operate at 6v? Should I eventually switch these out for two 6v coils?

Also found some mystery wires

And these were the only two terminals that didn't have anything connected to them. Any idea what they are?

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autolux
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Pictures.

Pictures.

  • img_3074.jpg
  • img_3075-1.jpg
  • img_3076.jpg
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schrader7032
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When you say the coils test

When you say the coils test at 5 ohms, how did you do that? Or did you mean 5K ohms. Electrickery is no my strong suit, but the 5K ohms that are needed is the secondaries (I think). Normally the coils are connected with a small jumper wire between them. So when you measure from plug cap to plug cap, the coils are being measured from one coil tower, through the jumper wire, to the other coil tower. In that case, you should see 10K ohms across the two coils. Check out Duane's picture:

A green/black wire supplies switched 12v power to one coil. A black wire out of the second coil goes to the condenser.

And yes, you should have two 6v coils. I'm guessing (again, I didn't do well in EE 101) that you're getting lower voltage out of the coils whenever they fire.

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

autolux
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Kurt, I measured the coils

Kurt,

I measured the coils across the primary & secondary terminals after I disconnected them completely. I tried to measure resistance across the entire circuit (from plug cap --> to plug cap) but I got no reading. Which I'm assuming is because my resistance was so high due to my left high tension lead being defective.

As far as the 12v coils go, I'm gonna take your advice and ditch em. I just ordered a pair of Bosch 6v super coils from Euro Motoelectrics in Denver (https://www.euromotoelectrics.com/ProductDetails.asp?ProductCode=BO-Coil...)

I have a set of new spark plug caps & wires, Air Filter, and ATU springs on the way as well.

Hopefully I'll have her running good again next week!

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mark_weiss
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12v and 6v coils need to be

12v and 6v coils need to be wired differently. 6v coils are wired in series (as is normal for two-coil airheads). The hot feed from the ignition circuit attaches to the '+' terminal of one coil, a jumper wire crosses from the first coil's '-' t the second coil's '+', and the second coil's ground terminal connects to the wire that goes to the breaker points (ground). This wires the two 6v coils end to end, making a virtual 12v, dual tower, ignition coil.

12v coils should have been wired in parallel. The hot wire would split and feed each coil separately. The point wire would also split and attach to each coil's ground side. An important consideration here is to ensure that each coil has sufficient resistance in order to avoid drawing excessive current which would cause rapid breaker point wear. The two coils wired side by side would remain effectively 12v, but their overall resistance in the circuit is halved. ie. If the circuit is supposed to have 4 ohms resistance, you would need to 8 ohm coils (same as working with hi fi speakers).

When wiring two components in series, as 6v coils are to be set up, their voltage rating is combined as is their resistance. When wiring in parallel, their voltage rating is unchanged but their average resistance is halved. If the two 12v coils were just substituted for 6v coils and the wiring unchanged, the effect would have been of a high resistance, 24v coil. That could probably kill a condenser too.

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